1. Field of the Invention
The present invention relates to a linear regulator to provide a regulated voltage to a load and particularly relates to frequency compensation for such a linear regulator.
2. Background of the Invention
Linear regulators are well-known devices that provide a regulated voltage to a load based on a source voltage and (usually) a reference voltage. FIG. 1 shows a conventional arrangement in which linear regulator 10 is connected to a source voltage V+ and provided with a reference voltage Vref so as to provide a regulated voltage to load 12.
To compensate for frequency-induced variations in current drawn by load 12, a load capacitor CL is often provided. Because there are often high fluctuations in the current drawn by load 12, however, a large value for CL is required, typically from 1 to 100 xcexcf. Such a large value is disadvantageous since large capacitors are large physically and also expensive.
As seen in FIG. 2, a conventional linear regulator 10 includes a bipolar device BP2 connected between the source voltage and the load so as to provide a regulated output voltage. The regulated output voltage is stabilized with a unity gain negative feedback amplification circuit through amplifier A1 which is provided with a reference voltage. A capacitive amplification circuit 13 includes a bipolar device BP1, amplifier A3 and capacitor Cm in a feedback relationship.
Although good results have been obtained with the linear regulator shown in FIG. 2, difficulties are still encountered. Most notably, the frequency roll off characteristics of the linear regulator shown at 10 are highly dependent on the actual value of the current drawn by load 12. Thus, for example, highest frequency roll off for linear regulator 10 depends on inherent resistive and capacitive effects of bipolar device BP2 (shown schematically at rxcfx80 and cxcfx80). In addition, the load capacitor CL actually includes a small series resistance Rs which introduces at least one additional zero into the frequency response of linear regulator 10. As a result of the additional zero, as well as the current dependence of system poles, it is easy to introduce instabilities in the linear regulator shown at 10 unless the current range of load 12 is small (for example, between 0 and 200 milliamps) and unless CL is a high quality capacitor such that its series resistance Rs is very small.
It is an object of the invention to provide a linear regulator whose frequency characteristics are less dependent on the amount of current drawn by load 12, and which exhibits a higher degree of frequency compensation than known linear regulators.
In one aspect, the invention is a linear regulator in which a capacitive amplification circuit includes a MOSFET device connected to the base of a bipolar output device so as to stabilize the current flow from the base to the output. Because a MOSFET device is used rather than the bipolar devices found in the prior art, a linear regulator according to the present invention exhibits frequency characteristics whose dependence is less than that of the prior art.
Thus, a linear regulator operable from a source voltage to provide a regulated voltage to a load includes a bipolar device connected between the source voltage and the load with an output of the bipolar device connected to output the regulated voltage, a feedback amplifier connected in negative feedback relationship between the output of the bipolar device and a reference voltage so as to provide a stabilized voltage, and a capacitor amplification circuit connected between the stabilized voltage and the output of the bipolar device. The capacitive amplification circuit includes a MOSFET device connected to a base of the bipolar device so as to stabilize current flow from the base to the output of the bipolar device. The capacitor amplification circuit includes an amplifier and a capacitor connected in feedback relationship with the output of the linear regulator, with an output of the amplifier stage providing a reference signal to the gate of the MOSFET device. Most preferably, a 1:n current mirror provides even greater current independence for the frequency characteristics of the linear regulator.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings.